This invention relates to delay lines for effecting desired time delays in the transmission of electromagnetic waves.
Transmission lines used to obtain pulse time delays are one class of structure known as delay lines. The line must be rather long even for small time delays since the electromagnetic waves propagate at a speed close to the speed of light. Special compact low-velocity lines have been developed to avoid this inconvenience. The most common type is a coaxial line, in which the inner conductor is a helix. The vast majority of the so-called "electric" delay lines are artificial transmission lines consisting of lumped capacitors and inductors. The limitations of physically realizable amplitude- and phase-transfer functions are such that the practical delays obtained do not exceed the order of a few pulse periods. Longer time delays are achieved with acoustic delay lines, employing acoustic wave propagation and electromechanical transducers at the input and output. See, for example, Electronic Engineers' Handbook, Donald G. Fink (ed.), 2d Edition, McGraw-Hill, (1982); and Introduction to Microwaves, Gershon J. Wheeler, Prentice-Hall, (1963).
Though very small, flexible and compact, a coaxial cable delay line can have problems at high frequencies in that it exhibits very high insertion loss. The actual amounts of delay required usually involve very long lengths of cable. Generation of power at these frequencies is extremely expensive and, therefore, this is an important factor.
Conventional metal waveguide delay lines are rigid copper tubes which are difficult to package and pose numerous installation problems. A problem with this type of delay line is that of dispersion. Dispersion is the phenomenon wherein different frequencies travel with different velocities. This type of delay line can provide a situation whereby, over a band of frequencies, there will be radically different values for the absolute delay.
Down-convertors with surface acoustic wave delay lines involve down-converting the micro/millimeter wave signal to a low-frequency acoustic signal which may be delayed using a surface acoustic wave delay line. This line will only work over a narrow band and is thus of limited use.
U.S. Pat. No. 4,463,329 discloses a dielectric waveguide of a shaped article having a core of polytetrafluoroethylene and having one or more layers of expanded, porous polytetrafluoroethylene overwrapped on or around the core.
U.S. Pat. No. 4,603,942 discloses a flexible waveguide for transmitting waves from a sensor mounted on a gimbal which includes a cable comprising an outer flexible sheath and a plurality of flexible polytetrafluoroethylene fibers bundled within the sheath and including a termination flange coupled to at least one end thereof, with the flange including a wedge-shaped plug and a tapered cavity engaging the end of the cable.